Keesom orientation forces

Keesom Orientation Forces. These forces result from the interaction of two permanent dipoles, with the hydrogen bond being the most important. Hydrogen bonds are stronger than dispersion or inductive forces. 

Keesom orientational force permanent dipole-permanent dipole interactions [45]. 

For a pair of identical molecules, it is noted in Eq. (13) that the first term determined with regard to the deformation polarizability is a so-called Debye inductive force , and the second term is generally called a Keesom orientational force between molecules when the dipole moment is considered in the intermolecular attractive system. 

Dispersion Forces Interaction forces between any two bodies of finite mass. Sometimes called van der Waals forces, they include the Keesom orientation forces between dipoles, Debye induction forces between dipoles and induced dipoles, and London (van der Waals) forces between two induced dipoles. Also referred to as Lifshitz—van der Waals forces. 

Keesom Orientation Forces. These forces result from the interaction of two permanent dipoles, of which the hydrogen bond is the most important. Hydrogen bonds are stronger than dispersion or inductive forces. If the two components have the same vapor pressure, separation can be achieved on the basis of several properties. These properties are (in the order of then-ease of separation) (1) difference in the functional groups, (2) isomers with polar functional groups, and (3) isomers with no functional groups. 

Weak, secondary forces, resulting from molecular dipoles, also act between materials. They are often classified according to the nature of the interacting dipoles. Keesom orientation forces act between permanent dipoles, London dispersion forces between transient dipoles, and Debye induction forces between a permanent and an induced dipole, see O Tables 2.1 and O 2.2. These are collectively known as van der Waals forces (but note alternative usage of this term, O Table 2.2), and occur widely between materials. They are much less dependent upon specific chemical structure than primary bonds. Indeed, dispersion forces are universal. They only require the presence of a nucleus and of extranuclear electrons, so they act between all atomic and molecular species. 

To reflect the contribution of the fundamental nature of the long-range interaction forces across the interface, it was suggested (Fowkes 1964) that surface free energies and work of adhesion may be expressed (O Eq. 3.11) by the sum of two terms the first one representative of London s dispersion interactions (superscript D) and the second representative of nondispersion forces (superscript ND), this latter include Debye induction forces, Keesom orientation forces, and acid—base interactions. 

There also exist dispersion, or London-van der Waals forces that molecules exert towards each other. These forces are usually attractive in nature and result from the orientation of dipoles, and may be dipole-dipole (Keesom dispersion forces), dipole-induced dipole (Debye dispersion forces), or induced dipole-induced dipole... 

Dipole-dipole (free rotation/ Keesom or orientation force) ... 

In the case of physical bonds (London dispersion, Keesom orientation, and Debye induction forces), the energy of interaction or reversible energy of adhesion can be directly calculated from the surface free energies of the solids in contact. 

Orientation Forces Keesom forces. See Dispersion Forces. 

Weaker secondary bonds act between molecules. Thus, below — 182°C, methane is a solid, the covalent molecules being held in a solid lattice be weak secondary bonds. These weak forces are associated with interactions between dipoles. Three different types of interaction have been described by London, Debye and Keesom, known respectively as dispersion, induction and orientation forces see Table 1 and Dispersion forces and Polar forces. The three types of interaction are often referred to collectively as van der Waals forces, as indicated in Table 1. However, it is necessary to note that some authors use the term van der Waals to refer exclusively to dispersion forces, the other two types being referred to as polar forces . Table 2. (The term dispersive is sometimes used by francophone authors writing in English where dispersion would be correct.)... 

The origin of forces between neutral symmetrical molecules, such as hydrogen (H2) or the inert gases (e.g. A, Ne), is not obvious. Because of the symmetry of the electron configuration, there cannot be any permanent dipole so, there can be neither dipole-dipole interactions (Keesom orientation interactions) nor dipole-molecule interaction (Debye induction interactions) (see Polar Forces). Further, there appears to be no Coulombic electrostatic interaction since they are electronically neutral overall, nor can there be any covalent bonding. Yet, there must be forces of some type between these molecules as the existence of liquid and solid hydrogen and argon demonstrate. 

Now let us examine the term of attraction. The force of attraction is due to three different physical effects the Keesom orientation effect, the Debye induction effect and the London dispersion effect. 

The form of interaction functiorrs such as those of Lennard-Jones, based on the model of Van der Waals forces irrvolving Keesom orientation effects, Debye induction and Lorrdon dispersion, which quickly decrease with distance beyorrd a certain distance between two molecules, the interaction can be negligible (for example, when the interaction is less than 5q/100)-This comes down to defming around each molecule a volume influence ... 

Orientation Forces. Besides the most basic non-polar interaction, dispersion forces, there are polar interactions between molecules of counterbodies, e.g. the dipole-dipole interaction (Keesom), the dipole-induced dipole interaction (Debye) and hydrogen bonding. The Keesom interaction (orientation) is temperature dependent and the energy is expressed as... 

Besides the most basic and predominant nonpolar interactions (dispersion forces), there are polarization or polar interactions between molecules of counter bodies, such as dipole-dipole interactions (Keesom 1922) and dipole-induced dipole interactions (Debye 1921). The essential difference between dispersion and polarization forces is that, while the former involve simultaneous excitation of both molecules, those for the latter involve only a passive partner. The Keesom orientation interaction energy between two molecules with permanent dipoles is temperature dependent and proportional to the dipole moments as follows ... 

The first hint that there are non-covalent interactions between uncharged atoms and molecules came from the observations of van der Waals (1873, 1881). These interactions came to be known as van der Waals forces. The interactions responsible for these became clear with the work of Keesom (1915, 1920, 1921), Debye (1920, 1921) and London (1930) as, respectively, interactions between two permanent dipoles (orientation forces), a permanent dipole and an induced dipole (induction forces) and a fluctuating dip>ole and an induced dipole (dispersion forces). While these three kinds of interaction have different origins, the interaction energies for all three vary as the inverse of the distance raised to the sixth power ... 

Attractive and Repulsive Forces. The force that causes small particles to stick together after colliding is van der Waals attraction. There are three van der Waals forces (/) Keesom-van der Waals, due to dipole—dipole interactions that have higher probabiUty of attractive orientations than nonattractive (2) Debye-van der Waals, due to dipole-induced dipole interactions (ie, uneven charge distribution is induced in a nonpolar material) and (J) London dispersion forces, which occur between two nonpolar substances. 

A dipole-dipole interaction, or Keesom force, is analogous to the interaction between two magnets. For non-hydrogen bonding molecules with fixed dipoles, these interactions are likely to influence the orientation of the molecules in the crystal. This is because, unlike the Debye force which is always attractive, the interaction between two dipoles is only attractive if the dipoles are properly oriented with respect to one another, as is the case with magnets. 

Van der Waals postulated that neutral molecules exert forces of attraction on each other which are caused by electrical interactions between dipoles. The attraction results from the orientation of dipoles due to any of (1) Keesom forces between permanent dipoles, (2) Debye induction forces between dipoles and induced dipoles, or (3) London-van der Waals dispersion forces between fluctuating dipoles and induced dipoles. (The term dispersion forces arose because they are largely determined by outer electrons, which are also responsible for the dispersion of light [272].) Except for quite polar materials the London-van der Waals dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

Induction (or Debye) and Orientation (or Keesom) force 0°+K which are the specific (or polar) properties of the van der Waals attraction exist in the presence of the dipole moment and (total) polarizability, resulting in specific (or polar) intermolecular attraction. 

In the first group of molecular compounds both partners possess a dipole moment and the compound can be considered as being produced thanks to the possibility of attaining a favourable mutual orientation of the dipoles (Keesom forces). Looked at in this way the numerous cases of association also belong to this group. These phenomena in which hydrogen dipoles, especially such as FH, OH, NH and (GH), play a part are discussed under the hydrogen bond . 

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